Digital content stored on Blu-ray Discs (BD) or Digital Versatile Disks (DVDs) or provided by digital video transmissions via broadcast, cable, satellite or IP is usually represented with 8 bits per channel. In the red/green/blue (RGB) format, this corresponds to 8 bits for each of the three primary color components, Red (R), Green (G) and Blue (B). In the YCbCr format, this corresponds to 8 bits for luminance (Y), 8 bits for the chrominance component Cb, and 8 bits for the chrominance component Cr. Cb and Cr may be down-sampled by 2 horizontally to create the 4:2:2 format and also vertically to create the 4:2:0 format.
The Human Visual System (HVS) can distinguish much more gray levels than the 256 levels offered by 8 bit data. Weber's Law states that the smallest step the eye can see is proportional to the absolute intensity, ΔI/I≈1.4. This means the eye is more sensitive to changes in levels at lower gray levels and significant banding or contouring is visible when the steps are not small enough. In order to produce a smooth gray scale ramp from black to white that has no visible banding or contouring effects, at least 1024 levels are required, corresponding to a bit depth of 10 bits per channel.
In view of the above-mentioned characteristic of the HVS, mainstream display technologies like LCD, Plasma and DLP have increased their bit-depth to 10-bits or more per channel. Also, recent versions (e.g., v1.3, v1.4) of HDMI, the popular digital interface used by consumer electronics devices worldwide, support transfer of video content with bit depths greater than 8 bits per channel. HDMI is used to transfer multimedia content from various audio-visual (AV) source devices like Blu-ray players, DVD players and set top boxes (STBs) to digital TVs.
However, as mentioned previously, commonly used consumer video formats still use 8 bits per channel. In order to display this data on a 10-bit or higher display, a typical video processor or converter appends extra bits to a lower bit depth pixel. For example, a converter can append 2 bits onto an 8-bit pixel value to create a 10-bit pixel value. However, simply appending bits does nothing to prevent the appearance of contouring or banding in smooth regions with slow variations where the individual steps are visible to the human eye. Conventionally, dithering, blurring, and smoothing techniques attempted to increase bit depth while reducing contouring artifacts. However, unintentional blurring and other undesirable results typically occurred. Therefore, a better way of processing image data would be desirable which uses the additional bit-depth offered by new digital interfaces and digital displays, to display images that are free from contouring artifacts while preserving the sharpness and detail.